Effect of Anions on the Changes in the Structure and Adsorption Mechanism of Zirconium Species at the Muscovite (001) – Water Interface

Multivalent cations primarily exist as polynuclear hydroxo and oxyhydroxo clusters and/or nanoparticles (NPs) in aqueous environments, where their interactions with mineral surfaces can be influenced by complexing anions. Here, we investigated the effect of background electrolyte anions (specifically ClO4-, Cl-, SO42-, and HPO42-) on the distribution of tetravalent Zr adsorbed on the negatively charged basal surface of muscovite mica. The experiments were conducted at constant pH (= 3), total Zr(IV) concentration (= 0.1 mM), and ionic strength (= 0.1 M). The Zr coverages measured by X-ray fluorescence (XRF) in ClO4- and Cl- systems were 1.3 and 2.1 Zr/AUC (where AUC = 46.72 Å2 is the area of the unit cell of the mica (001) surface), respectively, and mostly remained unchanged during the reaction time from 6 to 50 h. In these conditions, Zr adsorption occurred both as small NPs (with an average height of ~4 nm observed by ex situ atomic force microscopy, AFM) and in a ~2 nm-thick molecular layer (observed by in situ resonant anomalous X-ray reflectivity, RAXR). In comparison, higher Zr coverages that increased with reaction time (i.e., from ~4 to ~7 Zr/AUC from 6 to 50 h reactions) were observed in the SO42- system. Ex situ AFM revealed that the Zr uptake in this system occurred predominantly as NPs that were ca. 5 to 15 nm tall and 20 to 40 nm wide, but no evidence of the interfacial molecular-layer formation was observed by RAXR. In the HPO42– solution, all measurements unequivocally showed no significant Zr sorption, in stark contrast to the observations in the other solutions. Details of the Zr surface coverage and sorption modes are consistent with the known clustering of tetravalent Zr in solutions in the presence of these anions. These results demonstrate the significant impact of anions on the adsorption affinity and mechanism of Zr on the negatively charged mica surface.